A modified heat-moisture exchanger that incorporates a reflecting filter for use with partial rebreathing of exhaled volatile anaesthetics has been commercially available since the 1990s. The main advantages of the device are efficient delivery of inhaled sedation to intensive care patients and reduced anaesthetic consumption during anaesthesia. However, elevated arterial CO2 values have been observed with an anaesthetic conserving device compared with a conventional heat and moisture exchanger, despite compensation for larger apparatus dead space.